Infrared detectors are used in a variety of applications to provide an electrical output which is a useful measure of the incident infrared radiation. For example, quantum detectors are one type of infrared detector that are often used for night vision purposes in a variety of military, industrial and commercial applications. Quantum detectors generally operate at cryogenic temperatures and therefore require a cryogenic cooling apparatus. As a result, quantum detectors that operate at cryogenic temperatures can have a relatively complex design and generally consume significant amounts of energy.
Another type of infrared detector is a thermal detector. Thermal detectors are typically uncooled and therefore generally operate at room temperature. One type of thermal detector that has been developed and is becoming increasingly popular is a microbolometer-based, uncooled focal plane array. A focal plane array generally includes a plurality of pixel structures, each of which include a bolometer disposed upon a common substrate. Each bolometer includes a transducer element that has an electrical resistance that varies as a result of temperature changes produced by the incident infrared radiation. By detecting changes in the electrical resistance, a measure of the incident infrared radiation can be obtained. Since the design of a bolometer-based uncooled focal plane array is generally less complex than cryogenically cooled quantum detectors and since these uncooled focal plane arrays generally require significantly less energy than cryogenically cooled quantum detectors, bolometer-based uncooled focal plane arrays are being increasingly utilized.
Each pixel structure of a conventional uncooled focal plane array has a bolometer that includes an absorber element for absorbing infrared radiation and an associated transducer element having an electrical resistance that varies as its temperature correspondingly varies. Although the absorber and transducer elements can be separate layers of a multilayer structure, the absorber element and the transducer element may sometimes be the same physical element. In operation, infrared radiation incident upon the absorber element will heat the absorber element. Since the absorber element and the transducer element are in thermal contact, the heating of the absorber element will correspondingly heat the transducer element, thereby causing the electrical resistance of the transducer element to change in a predetermined manner. By measuring the change in electrical resistance of the transducer element, such as by passing a known current through the transducer element, a measure of the incident radiation can be obtained.
In order to permit the bolometer to be responsive to changes in the incident infrared radiation, the bolometer is generally designed to minimize thermal loss to the substrate. Thus, the bolometers of conventional focal plane arrays have separated the absorber and the transducer elements from the substrate so as to substantially thermally decouple the relatively massive substrate from the pixel. In this regard, each bolometer generally includes two or more legs that support the absorber and transducer elements above the substrate. The legs can extend between the absorber and transducer elements and the substrate, or the legs can connect the absorber and transducer elements to pillars or the like that support the absorber and transducer elements above the substrate.
In order to provide thermal contact between the absorber and the transducer elements while electrically insulating the transducer element from the absorber element, the bolometer also generally includes a thermally conductive, electrically insulating layer disposed between the absorber element and the transducer element. In addition, the bolometer typically includes another insulating layer disposed on the surface of the bolometer facing the substrate which serves to structurally support the other layers and to protect the other layers during the fabrication process. See, for example, U.S. Pat. Nos. 5,286,976; 5,288,649 and 5,367,167 which describe the pixel structures of conventional bolometer-based focal plane arrays, the contents of each of which are incorporated herein by reference.
In order to further improve the performance of conventional pixel structures, each bolometer can include a reflector disposed upon the surface of the substrate underlying the absorber and transducer elements. As such, infrared radiation that is incident upon the bolometer, but that passes through and is not absorbed by the absorber element, will be reflected by the reflector back towards the absorber element. At least a portion of the reflected radiation will therefore be absorbed by the absorber element during its second pass through the absorber element, thereby increasing the percentage of the incident radiation that is absorbed by the absorber element.
In operation, infrared radiation incident upon the pixel structure will be absorbed by the absorber element of the bolometer and the heat generated by the absorbed radiation will be transferred to the transducer element. As the transducer element heats in response to the absorbed radiation, the electrical resistance of the transducer element will change in a predetermined manner. In order to monitor the change in resistance of the transducer element and, therefore, to indirectly measure the infrared radiation incident upon the bolometer of the pixel structure, circuitry is generally formed upon the underlying substrate. The circuitry is generally electrically connected to the transducer element via a pair of conductive paths or traces defined by or upon the legs, pillars or the like that support the absorber and transducer elements above the surface of the substrate. By passing a known current through the transducer element, the change in electrical resistance of the transducer element can be measured and a corresponding measure of the incident infrared radiation can be determined.
While bolometer-based focal plane arrays having a plurality of pixel structures as described above are extremely useful, these conventional focal plane arrays have several disadvantages. Most notably, for the bolometer of each pixel structure, the characteristics of the absorber element and the transducer element cannot be separately optimized since the absorber and transducer elements are included within the same multilayer structure that is supported above the surface of the substrate. In this regard, in order to optimize the performance of the bolometer of a focal plane array, the absorption characteristics of the bolometer should be maximized while the thermal loss to the substrate should be minimized. In particular, the absorption characteristics are preferably maximized and the thermal loss to the substrate is preferably minimized in order to increase the sensitivity of the bolometer. As a secondary consideration, the thermal mass is also preferably reduced in order to decrease the time constant of the bolometer. Since both the absorber element and the transducer element are included within the same multilayer structure and, in some instances, are the same physical layer, design changes made to increase the absorption characteristics of the bolometer generally also disadvantageously increase the thermal mass of the bolometer, while design changes made to decrease the thermal mass of the bolometer generally disadvantageously reduce the absorption characteristics of the bolometer. For example, increases in the size of the absorber layer of the bolometer of a conventional pixel structure that are made to increase its absorption characteristics will also disadvantageously increase the thermal mass of the bolometer. Likewise, the reduction of the thermal mass of the bolometer of a conventional pixel structure by thinning one or more layers of the bolometer will disadvantageously decrease the absorption characteristics of the bolometer.
Since the pixel structures are generally disposed in an array, it is desirable that the absorber layer of the bolometer of each pixel structure be as large as possible and approximate the overall size of the pixel structure as closely as possible in order to maximize the fill factor. Unfortunately, the legs, pillars or the like that support the absorber and transducer elements of conventional bolometer above the surface of the substrate generally extend from edge portions of the absorber and transducer elements to the substrate. Due to the design of these conventional bolometers, the absorber layer is therefore typically unable to completely cover the legs, pillars or the like.
While conventional focal plane array having pixel structures with bolometers as described above have been sufficient for most applications in the past, these conventional focal plane arrays are typically unable to meet the heightened requirements proposed by a number of future applications that demand increased performance and perhaps decreased size. In particular, as the pixel density and performance requirements of focal plane arrays are increased, the compromises that have been made in the past in order to balance the absorption characteristics, the thermal loss and the thermal mass of a conventional bolometer become unworkable. In addition, concerns relating to the costs to manufacture focal plane arrays and the size of the resulting focal plane arrays are driving focal plane arrays to be even smaller, thereby correspondingly placing even more importance upon maximizing the fill factor of the pixel structures. For the reasons described above, conventional focal plane arrays having a plurality of pixel structures, each of which includes a bolometer having both an absorber element and a transducer element supported above the surface of a substrate do not appear to be able to meet the performance requirements demanded by some modern applications.